JP5029361B2 - Hot-rolled steel sheet, cold-rolled steel sheet and methods for producing them - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet and a cold-rolled steel sheet having ultrafine crystal grains, and methods for producing them. Specifically, hot-rolled steel sheets and cold-rolled steel sheets excellent in mechanical strength, workability and thermal stability suitable for materials used for automobiles, home appliances, machine structures, constructions, etc., and their production Regarding the method.

  Steel sheets used as materials for automobiles and other transport machinery and various industrial machinery structural members not only have excellent mechanical properties such as strength, workability, and toughness, but also weldability during assembly. In some cases, corrosion resistance during use is required. In order to comprehensively improve the mechanical properties of the steel sheet, it is effective to refine the structure of the steel sheet. Therefore, many methods for refining the structure of the steel sheet have been proposed.

  Summarizing the means for refining the structure in the prior art, it is (i) a large rolling reduction method, (ii) a controlled rolling method, (iii) an alloy element addition method, or a combination thereof.

  (I) The large rolling reduction method increases the rolling reduction to about 50% or more, accumulates large strain by one-pass rolling, and then transforms from austenite to fine ferrite, or uses large strain. This is a technique for recrystallizing relatively coarse ferrite into fine ferrite. According to such a method, after heating to a temperature of about 1000 ° C. or lower and then rolling under a large pressure in a low temperature region of about 700 ° C., an ultrafine ferrite structure of 1 to 3 μm can be obtained. However, this method is difficult to realize industrially, and since the fine ferrite structure easily grows by heat treatment, the welded part softens when welded, or the expected mechanical properties are obtained when hot-dip Zn plating is applied. Has problems such as losing.

(Ii) The controlled rolling method is generally a method of cooling after performing multi-pass rolling at a temperature of about 800 ° C. or higher and a rolling reduction per rolling of 20 to 40% or less. There are many methods such as a method in which the rolling temperature is set to a narrow temperature range near the Ar ₃ point, a method of shortening the time between rolling passes, and a method of dynamically recrystallizing austenite by controlling the strain rate and temperature. It is disclosed. However, studies on cooling after rolling have not been sufficiently conducted. It is said that it is preferable to perform water cooling immediately after rolling, but cooling immediately after the rolling starts 0.2 seconds or more after rolling, and the cooling rate is at most about 250 ° C./second. In such a method, the ferrite crystal grain size of a low-carbon steel having a simple composition is only about 5 μm. Therefore, the mechanical properties cannot be sufficiently improved.

  (Iii) The alloy element addition method promotes refinement of ferrite crystal grains by adding a small amount of an alloy element that suppresses recrystallization and recovery of austenite. Alloy elements such as Nb and Ti form carbides or segregate at grain boundaries to suppress austenite recovery and recrystallization, so that austenite grains after hot rolling are refined, The ferrite crystal grains obtained by transformation are also refined. The alloy element addition method (iii) is often used in combination with the above-described large rolling method (i) or the controlled rolling method (ii). This alloy element addition method (iii) also has the effect of suppressing ferrite grain growth during heat treatment. However, although the crystal grain size of ferrite is reduced, there is a problem that the volume fraction of ferrite is reduced, and it is not sufficient to suppress grain growth in the welding of ultrafine ferrite grains and hot-dip Zn plating process. is there. Therefore, applicable steel types are limited. Further, the raw material cost increases by the amount of the alloy element to be added.

There is Patent Document 1 as a prior art document that refers to these (i) large rolling rolling methods, (ii) controlled rolling methods, and (iii) methods of adding alloy elements. Here, processing with a total rolling reduction of 50% or more is performed once or twice within one second in a temperature range of Ar ₁ + 50 ° C. to Ar ₃ + 100 ° C., and in a temperature range of 600 ° C. or more after completion of processing. A method of performing forced cooling at a cooling rate of 20 ° C./second or more is disclosed.

  Patent Document 2 discloses that the first stand entry side and the last stand where the reduction in the dynamic recrystallization temperature range is performed in a reduction pass of five or more stands and the reduction is applied in the dynamic recrystallization temperature range. A method of setting the temperature difference on the outlet side to 60 ° C. or less is disclosed.

JP 59-205447 A Japanese Patent Laid-Open No. 11-152544

  However, even if a steel sheet having a fine crystal structure is obtained by these methods, the thermal stability of the structure is low. Therefore, even if the structure is refined and mechanical properties are improved, if the steel sheet is subsequently welded or hot-plated on the steel sheet, the crystal grains are easily formed by the heat applied during welding or the heat applied during the hot-dipping process. There is a problem that the mechanical properties are extremely deteriorated. In addition, even when these hot-rolled steel sheets are subjected to cold rolling and heat treatment to form thin steel sheets, there is a problem that the crystal grains are easily coarsened by the heat treatment and it is not possible to obtain a cold-rolled steel sheet having a fine structure. .

  The present invention provides a hot-rolled steel sheet and a cold-rolled steel sheet that have ultrafine crystal grains and are excellent in thermal stability and mechanical properties that can withstand the heat of welding and hot dipping processes, and methods for producing the same. For the purpose.

As a result of various studies and experiments on the mechanical properties and thermal stability of the fine ferrite grain structure, the present inventors have found that both mechanical properties and thermal stability are excellent. (a) keeping the average crystal grain size of ferrite within a certain range; and (b) increasing rate X (μm / min) of the average crystal grain size D (μm) of ferrite at a temperature in the vicinity of 700 ° C. just below _one point of A. And the upper limit of the product D · X (μm ² / min) of the average crystal grain size D (μm) was found to be most important. In addition, in order to obtain better thermal stability, (c) it is necessary to keep the ferrite crystal grain size distribution within a certain range or to leave no strain due to rolling in the ferrite crystal grains. I found it preferable. It has also been found that when such a steel sheet is heat-treated after cold rolling, it again has a thermally stable and fine ferrite crystal grain structure as described above. Various studies and experiments were also conducted on (d) a new method for producing hot-rolled steel sheets and cold-rolled steel sheets having such a structure and characteristics. Further, regarding the welded member, it is preferable to define the hardness balance of the welded part in (e) fusion welding, and (f) it is preferable to aim at the hardness balance and suppression of embrittlement in resistance welding. I found out.

  Hereinafter, in (a) to (f), the knowledge and examination / experimental results according to the present invention will be described in detail.

(a) To keep the average grain size of ferrite within a certain range

The strength increases as the crystal grain size of ferrite decreases, but it has been found that if the crystal grain size becomes too small, the driving force of grain growth by grain boundary energy increases, which promotes grain growth at high temperatures. Specifically, when the average crystal grain size is less than 1.2 μm, it becomes difficult to suppress grain growth at high temperature, and conversely, the average crystal grain size is 2.7 + 5000 / (5 + 350 · C + 40 · Mn) When the value exceeds ² μm or 7 μm, and for cold-rolled steel sheet, 5.0−2.0 · Cr + 5000 / (5 + 350 · C + 40 · Mn) ² μm and 9.3 μm It has been found that if any value is exceeded, improvement in mechanical properties due to miniaturization cannot be sufficiently expected. Therefore, in order to achieve both mechanical properties and thermal stability, 1.2 μm is adopted as the lower limit of the average crystal grain size of ferrite, and 2.7 + 5000 / (5 + 350 · C + 40) is used as the upper limit for hot-rolled steel sheets.・ Mn) The smaller value of ² μm and 7 μm. For cold-rolled steel sheet, the smaller value of 5.0−2.0 · Cr + 5000 / (5 + 350 · C + 40 · Mn) ² μm and 9.3 μm It is necessary to adopt.

(b) A Regarding the upper limit of the product D · X of the average grain size D of ferrite and the average grain size D of ferrite at a temperature near 700 ° C just below _one point, the grain growth rate of ferrite grains at high temperature is Increases with increasing temperature. In general, a temperature range where problem grain growth of ferrite occurs by welding or hot-dip plating process is the temperature range from just under ₁ point A (730 ° C. vicinity) to the vicinity of A ₃ points, grain growth rate of the ferrite in the temperature range It changes a lot. However, since it was found that the temperature characteristic of the grain growth rate of the steel sheet having the average crystal grain size of ferrite in the range of (a) is determined by the ferrite grain growth rate at a temperature in the vicinity of 700 ° C., 700 ° C. The upper limit is the grain growth rate of ferrite at a nearby temperature, that is, the product D · X (μm ² / min) of the average crystal grain size increase rate X (μm / min) and the average crystal grain size D (μm). It was found that no problem occurs even when heated to a higher temperature in the welding or hot dipping process. As a result of the experiment, it was also found that the product D · X needs to be set to 0.1 μm ² / min or less. Incidentally, the product D · X is preferably from 0.07 .mu.m ² / min, more preferably 0.05 .mu.m ² / min or less.

(c1) Keeping the ferrite grain size distribution within a certain range and keeping the ferrite grains free from strain due to rolling The ferrite grain size distribution and the strain inside the ferrite grains are high. It is closely related to grain growth. Grain growth at high temperatures is caused by the grain boundary energy and intra-granular distortion as driving forces. Therefore, when relatively large ferrite crystal grains are mixed in a fine ferrite structure, the large ferrite crystal grains are easily integrated with the surrounding fine ferrite crystal grains using the grain boundary as a driving force. Further, if there is strain in the ferrite crystal grains, adjacent ferrite crystal grains are easily integrated with each other using the strain in the grains as a driving force. In this way, grain growth proceeds rapidly. For this reason, in order to prevent rapid progress of grain growth, in addition to the refinement of ferrite crystal grains, the crystal grain size distribution of ferrite is 80% or more in the range of 1/3 to 3 times the average grain size. It is preferable to keep the grains of the particles. This crystal grain size distribution is measured within a predetermined depth from the plate surface or within a range of 100 μm from the depth. As will be described later, the crystal grain size of the steel sheet according to the method of the present invention changes in the direction of the plate thickness, but such a gradual change in the crystal grain size in the direction of the plate thickness affects the grain growth. It is because it does not give. Further, the intragranular dislocation density showing the strain in the ferrite crystal grains is preferably 10 ⁹ / cm ² or less, and more preferably 10 ⁸ / cm ² or less. Further, the shape of the ferrite grains is preferably equiaxed.

(c2) About the distribution of ferrite grain size in the plate thickness direction The mild ferrite grain size distribution in the plate thickness direction, which becomes finer from the center of the steel plate to the surface layer of the steel plate, is the processing of the steel plate such as hole expandability and bendability. It is preferable for improving the property. In addition, the finer ferrite structure in the surface layer part improves the surface treatment properties such as chemical conversion treatment and plating properties of the steel sheet. Therefore, for the hot-rolled steel sheet, the average crystal grain size Ds (μm) at a depth position of 1/16 of the sheet thickness from the steel sheet surface, and the average crystal grain diameter D (at a depth position of 1/4 of the sheet thickness from the steel sheet surface). [mu] m) and the average grain size Dc ([mu] m) in the central portion of the plate thickness preferably satisfy the relationship of Ds≤0.75Dc and D≤0.9Dc, and Ds≤0. It is preferable to satisfy the relationship of 9Dc.

(d) A new method for producing a hot-rolled steel sheet having the structure and characteristics of (a) to (c) above. By adopting rolling in a high temperature range as follows, rolling is easy and high productivity The industrial method can be provided.

First, multi-pass hot rolling is started from the austenite temperature range, and the final rolling pass is finished at a high temperature of Ar ₃ points or higher and 780 ° C. or higher. At this time, strain is accumulated in the austenite crystal grains.

  And cooling to the temperature of 720 degrees C or less is completed within 0.4 second immediately after completion | finish of hot rolling. At this time, since the release of this strain is suppressed during cooling, the strain is accumulated in the austenite grains, and the transformation from austenite to ferrite is activated only when the temperature reaches 720 ° C. or less. A large number of ferrite crystal grains are generated using the accumulated strain as a nucleus to form a fine ferrite structure. In this method, it is possible to suppress the release of shear strain introduced into the steel plate during hot rolling by friction between the steel plate surface and the rolling roll surface, so that it is more in the portion closer to the surface than the center of the plate thickness. Ferrite nuclei are generated.

  Furthermore, after that, the temperature is maintained at 600 to 720 ° C. for 2 seconds or more. As a result, a desired ferrite structure that is fine and distributed in a narrow range of the crystal grain size can be obtained, and strain can be prevented from remaining in the fine ferrite structure after transformation. In addition, due to the change in the amount of ferrite nucleation in the plate thickness direction described above, a structure having a gentle grain size gradient from the plate thickness center to the surface is generated.

In addition, the cooling conditions immediately after completion | finish of hot rolling need to complete the cooling to the temperature of 720 degrees C or less within 0.4 second as above-mentioned. Conventionally, even the fastest one started cooling after 0.2 seconds or more immediately after the end of rolling, and the cooling rate was about 250 ° C./second at most. Taking a low carbon steel with an Ar ₃ point of 800 ° C. as an example, even if the hot rolling of the low carbon steel is terminated at the Ar ₃ point, while the cooling is performed from 800 ° C. to 720 ° C., Since 0.52 seconds or more had elapsed, it was difficult to complete the cooling to a temperature of 720 ° C. or less within 0.4 seconds.

After cold rolling the hot-rolled steel sheet having the above structures (a) to (c) and then heat-treating it at a temperature lower than the temperature at which it becomes an austenite single phase (Ac ₃ ), the fine-grained ferrite structure having the above characteristics is obtained again. Become. This is because (1) when the processed ferrite is recrystallized during the heat treatment after cold rolling, it occurs on the ferrite grain boundary during hot rolling where a large amount of its nuclei exist, and thus a large number of ferrite nuclei are generated. (2) At the same time, a large number of austenite is also generated on the ferrite grain boundary during hot rolling, and this is considered to be due to the suppression of the growth of ferrite nuclei. As a result, the ferrite grain size after heat treatment is almost the same as the ferrite grain size at the time of hot rolling, or only 1 to 3 μm larger, and a structure that inherits the characteristics at the time of hot rolling can be obtained. Therefore, if there is a distribution in the thickness direction of the ferrite grain size at the stage of the hot rolled steel sheet as in the present invention, a similar distribution in the thickness direction of the ferrite crystal grain size appears even after cold rolling and heat treatment. The heat treatment temperature may be Ac ₁ or lower, but it takes a long time to recrystallize the processed ferrite. At a temperature at which Ac becomes an austenite single phase of ₃ points or more, the structure is easily coarsened.

(e) Defining the hardness balance of welds in fusion welding In arc welding, where heat input during welding is large, it is difficult to cause grain growth during welding from the viewpoint of preventing softening of HAZ (heat affected zone). Needless to say, it is preferable to create a structure having high thermal stability. Furthermore, in order to ensure the workability of the member after welding, it is preferable to improve the melt weldability by defining the hardness balance of the welded portion. That is, with respect to chemical composition, Ceq (I) = C + Mn / 6 + Si / 24 + Cr / 5 + Mo / 4 + Ni / 40 + V / 14 is defined as 0.06 to 0.6% by defining the carbon equivalent Ceq (I) as 0.06 to 0.6%. A weld zone having excellent weldability can be obtained. Melt weldability refers to the maximum hardness of the weld and the hardness of the base metal obtained by using a welding method that proceeds while forming and solidifying the molten pool continuously, such as arc welding and laser welding. Or the difference in hardness of the softest part of the welded portion becomes small, and embrittlement of the welded portion is suppressed, and the workability of the member after welding can be secured.

(f) About resistance hardness balance and resistance to embrittlement in resistance welding Even in resistance welding where welding is performed by energization heating to the base metal, a highly thermally stable structure that does not easily cause grain growth during welding. It goes without saying that it is preferable to make Furthermore, it is preferable to achieve a hardness balance and suppression of embrittlement of the weld. That is, regarding the chemical composition, the carbon equivalent Ceq (II) defined by C ≦ 0.17% and Ceq (II) = C + Mn / 100 + Si / 90 + Cr / 100 is defined as 0.03 to 0.20%, In order to obtain a sufficient nugget (molten joint) for securing the joint strength in a wide range of welding conditions, Rsp = 13.5 × (Si + Al + 0.4Mn + 0.4Cr) +12.2 By setting the index Rsp to 45 or less, a welded portion having excellent resistance weldability can be obtained. In addition, resistance weldability means the characteristic which can ensure sufficient joint intensity | strength (what is called the maximum breaking load at the time of a button fracture) in a wide range of welding conditions.

  The present invention has been completed on the basis of such findings and examination / experimental results. The gist of the present invention is the hot rolled steel sheet of the following (1), (2), (4) to (7) and (9) to (11), and (3) to (6) and (8). To (11) cold-rolled steel sheet, (12) and (14) hot-rolled steel sheet manufacturing method, and (13) and (14) cold-rolled steel sheet manufacturing method. Hereinafter, these are referred to as the present inventions (1) to (14), respectively. The present inventions (1) to (14) may be collectively referred to as the present invention.

  The carbon steel or low alloy steel used in the present invention preferably contains C: 0.01 to 0.25%, and further Si, Mn, Al, P, Ti, Nb, V, Cr, Cu , Mo, Ni, Ca, REM, or B may be included.

(1) By mass%, C: 0.01 to 0.25%, Si: 3% or less, Mn: 3% or less, Al: 3% or less, P: 0.5% or less, Ti: 0 to 0. 3%, Nb: 0 to 0.1%, V: 0 to 1%, Cr: 0 to 1%, Cu: 0 to 3 %, Ni: 0 to 1%, Mo: 0 to 1%, and Ca + REM + B: 0 It has a chemical composition consisting of 0.005% and the balance Fe and impurities, a steel sheet composed of ferrite carbon steel or low alloy steel containing not less than 50 vol%, a depth from the surface of the steel sheet thickness 1/4 of The average crystal grain size D (μm) of ferrite at the vertical position satisfies the following formulas (1) and (2), and the average crystal grain size of ferrite at a depth of 1/4 of the plate thickness from the steel sheet surface The increase rate X at 700 ° C. (μm / min) and the average crystal grain size D (μm) satisfy the following formula (3): Hot-rolled steel sheet, wherein the door.
1.2 ≦ D ≦ 7 (1) Formula D ≦ 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² (2) Formula D · X ≦ 0.1 (3) Formula where C and Mn are the elements of steel The content (unit: mass%) is shown.

1.2 ≦ D ≦ 7 (1) Formula D ≦ 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² (2) Formula D · X ≦ 0.1 (3) Formula where C and Mn are the elements of steel The content (unit: mass%) is shown.

  (2) The area ratio of the ferrite crystal grains at the position where the crystal grain diameter d (μm) satisfies the following formula (4) at the depth position of ¼ of the sheet thickness from the steel sheet surface is 80%. It is the above, The hot-rolled steel plate of Claim 1 characterized by the above-mentioned.

D / 3 ≦ d ≦ 3D (4) where D is the average ferrite crystal at a depth of 1/4 of the plate thickness from the steel plate surface. The particle size (μm) is shown.

(3) By mass%, C: 0.01 to 0.25%, Si: 3% or less, Mn: 3% or less, Al: 3% or less, P: 0.5% or less, Ti: 0 to 0. 3%, Nb: 0 to 0.1%, V: 0 to 1%, Cr: 0 to 1%, Cu: 0 to 3 %, Ni: 0 to 1%, Mo: 0 to 1%, and Ca + REM + B: 0 It has a chemical composition consisting of 0.005% and the balance Fe and impurities, a steel sheet composed of ferrite carbon steel or low alloy steel containing not less than 50 vol%, a depth from the surface of the steel sheet thickness 1/4 of The average crystal grain size D (μm) of ferrite at the thickness satisfies the following formulas (5) and (6), and the average crystal grain size of ferrite at a depth of 1/4 of the plate thickness from the steel sheet surface The increase rate X (μm / min) at 700 ° C. and the average crystal grain size D (μm) satisfy the following formula (3):
1.2 ≦ D ≦ 9.3 (5) Formula D ≦ 5.0−2.0 · Cr + 5000 / (5 + 350 · C + 40 · Mn) ²・ ・ (6) Formula D ・ X ≦ 0.1 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (3) The ratio of the area occupied by ferrite at the position of the ferrite crystal grains having a crystal grain size d (μm) satisfying the following formula (4) at a depth of 1/4 of the thickness is 80% or more. Cold rolled steel sheet.
D / 3 ≦ d ≦ 3D (4) where C, Cr and Mn are the contents of each element in the steel (unit: mass%) Indicates.

D / 3 ≦ d ≦ 3D (4) where C, Cr and Mn are the contents of each element in the steel (unit: mass%) Indicates.

  (4) In the steel sheet of any of (1) to (3) above, as a second phase other than ferrite, the volume ratio is less than 50% bainite, less than 30% pearlite, less than 5% granular cementite, 5 1 or 2 or more types selected from the group consisting of less than 3% martensite and less than 3% retained austenite, and a yield ratio of 0.75 or more. Rolled steel sheet or cold rolled steel sheet.

  (5) In the steel sheet of any of the above (1) to (3), the second phase other than ferrite contains 5 to 40% martensite by volume ratio, and the yield ratio is less than 0.75. A hot-rolled steel plate or a cold-rolled steel plate characterized by being.

  (6) In the steel sheet of any one of the above (1) to (3), the second phase other than ferrite contains 3 to 30% residual austenite by volume ratio, and has a tensile strength TS (MPa) and the total strength. A hot-rolled steel sheet or a cold-rolled steel sheet characterized in that the product TS × El with the elongation El (%) is 18000 (MPa ·%) or more.

  (7) Average crystal grain size Ds (μm) at a depth position of 1/16 of the plate thickness from the steel plate surface, Average crystal grain size D (μm) at a depth position of 1/4 of the plate thickness from the steel plate surface, (1), (2), (4) characterized by satisfying the relationship of Ds ≦ 0.75 Dc and D ≦ 0.9 Dc between the average crystal grain diameters Dc (μm) in the central portion of the thickness ), (5) and (6).

  (8) Between the average crystal grain size Ds (μm) at a depth position of 1/16 of the plate thickness from the steel sheet surface and the average crystal grain size Dc (μm) at the center of the plate thickness, D ≦ 0.9 Dc The cold-rolled steel sheet according to any one of (3) to (6), wherein the relationship is satisfied.

  (9) The hot rolling according to any one of (1) to (8) above, wherein the carbon equivalent Ceq (I) defined by the following formula (7) is 0.06 to 0.6%: Steel plate or cold rolled steel plate.

Ceq (I) = C + Mn / 6 + Si / 24 + Cr / 5
+ Mo / 4 + Ni / 40 + V / 14 (7) Formula Here, the element symbol in the formula indicates the content (unit: mass%) of each element in the steel.

  (10) The C content is 0.17% by mass or less, the carbon equivalent Ceq (II) defined by the following formula (8) is 0.03 to 0.20%, and the following ( 9) The hot-rolled steel sheet or cold-rolled steel sheet according to any one of (1) to (8) above, wherein the index Rsp of the base material resistance defined by the formula is 45 or less.

Ceq (II) = C + Mn / 100 + Si / 90 + Cr / 100 (8) Formula Rsp = 13.5 × (Si + Al + 0.4Mn + 0.4Cr) +12.2 (9) where the element symbol in the formula Indicates the content of each element in steel (unit: mass%).

  (11) A hot-rolled hot-rolled steel sheet comprising a coating layer of Zn, Al, Zn-Al alloy or Fe-Zn alloy on the surface of the hot-rolled steel sheet according to any one of (1) to (10). Or cold rolled steel sheet.

(12) A method of producing a hot-rolled steel sheet by multi-pass hot rolling a slab made of carbon steel or low alloy steel, and ending the final rolling pass at a temperature of Ar ₃ points or higher and 780 ° C. or higher, Then, after cooling to 720 ° C. or less within 0.4 seconds at a cooling rate of 400 ° C./second or more, the temperature is maintained at 600 to 720 ° C. for 2 seconds or more, (1), (2 ), (4), (5), (6), (7), (9), (10) and (11).

  (13) The hot-rolled steel sheet obtained by the method of (12) is pickled, cold-rolled at a rolling rate of 40 to 90%, and then heat-treated at a temperature of 900 ° C. or lower. (3), (4), (5), (6), (8), (9) and the manufacturing method of the cold-rolled steel plate in any one of (10).

  (14) The hot-rolled steel sheet obtained by the above method (12) is pickled or cold-rolled at a rolling rate of 40 to 90% after pickling and then hot-plated in a continuous hot dipping line. The method for producing a hot-rolled hot-rolled steel sheet or hot-rolled cold-rolled steel sheet as described in (11) above.

  According to the present invention, a hot-rolled steel sheet and a cold-rolled steel sheet having ultrafine crystal grains, excellent in thermal stability and mechanical properties capable of withstanding the heat of welding and hot dipping processes, and methods for producing them. Can be provided.

  Below, the ultra-fine grain hot-rolled steel sheet according to the present invention will be described. Hereinafter, “%” display of the content of each chemical component means “mass%”.

(A) About chemical composition C:
C is an element useful for promoting the refinement of ferrite crystal grains because it can lower the transformation temperature from austenite to ferrite and lower the finishing temperature of hot rolling. Moreover, it is an element for ensuring strength. For this reason, it is preferable to make it contain 0.01% or more. Moreover, in order to further promote the refinement of ferrite crystal grains, the content is preferably 0.03% or more. However, if it is contained excessively, ferrite transformation after hot rolling is delayed, the volume fraction of ferrite is lowered, and weldability is deteriorated, so it is preferably made 0.25% or less. In order to improve the workability of the weld zone, the C content is preferably 0.17% or less, and more preferably 0.15% or less.

Si:
Si is preferably contained for the purpose of improving the strength. However, if excessively added, the ductility is remarkably deteriorated and the problem of surface oxidation during hot rolling occurs. Therefore, the content is preferably 3% or less. Preferably it is 2% or less, More preferably, it is 1.8% or less. The lower limit may be an impurity level, but when residual austenite is generated in the ferrite structure, it is preferable to contain 1% or more in terms of the total amount of Si + Al.

Mn:
Mn is preferably contained to ensure strength. Moreover, since the transformation temperature from austenite to ferrite can be lowered and the finishing temperature in hot rolling can be lowered, it is preferably contained in order to promote refinement of ferrite crystal grains. However, if it is contained excessively, ferrite transformation after hot rolling is delayed and the volume fraction of ferrite is lowered, so the content is preferably made 3% or less. More preferably, it is 2.7% or less. The lower limit may be an impurity level, but when added for the purpose of improving the strength, it is preferable to contain 0.5% or more. Moreover, when producing | generating a retained austenite in a ferrite structure, it is preferable to contain 0.5% or more, and it is more preferable to contain 0.8% or more. Further, when martensite is generated in the ferrite structure, the total amount of Si + Mn is preferably 1% or more, and more preferably 1.5% or more.

Al:
Al may be added to improve ductility. However, if excessively contained, the austenite at high temperature becomes unstable, and it is necessary to excessively increase the finishing temperature in hot rolling, and it becomes difficult to achieve stable continuous casting. The following is preferable. The lower limit may be an impurity level, but when residual austenite is generated in the ferrite structure, it is preferable to contain 1% or more in terms of the total amount of Si + Al.

P:
P may be added to increase the strength. However, if excessively contained, embrittlement due to grain boundary segregation occurs. Therefore, when added, the content is preferably 0.5% or less. More preferably, it is 0.2% or less, More preferably, it is 0.1% or less. The lower limit may be an impurity level. Usually, about 0.01% is mixed in the steelmaking stage.

Ti:
Ti precipitates as carbide or nitride to increase the strength, and this precipitate suppresses the coarsening of austenite and ferrite to promote the refinement of crystal grains during hot rolling, and during the heat treatment In order to suppress grain growth, it may be added. However, if excessively contained, a large amount of coarse Ti carbide or nitride is generated at the time of heating before hot rolling to inhibit ductility and workability, so the content is preferably 0.3% or less. . In order to facilitate the formation of ferrite, the total amount of Ti + Nb is preferably 0.1% or less, more preferably 0.03% or less, and even more preferably 0.01% or less. The lower limit may be an impurity level. Generally about 0.001% is mixed on steelmaking.

Nb:
Nb precipitates as carbide or nitride to increase the strength, and this precipitate suppresses the coarsening of austenite and ferrite to promote the refinement of crystal grains during hot rolling, and during the heat treatment In order to suppress grain growth, it may be added. However, if excessively contained, a large amount of coarse NbC is generated at the time of heating before hot rolling to inhibit ductility and workability, so the content is preferably 0.1% or less. In order to facilitate the formation of ferrite, the total amount of Ti + Nb is preferably 0.1% or less, more preferably 0.03% or less, and still more preferably 0.01%. The lower limit may be an impurity level. Generally about 0.001% is mixed on steelmaking.

V:
V precipitates as a carbide to increase the strength, and this precipitate may be added to suppress the coarsening of ferrite and promote the refinement of crystal grains. However, for the same reason as Ti and Nb, the ductility and workability are inhibited, so the content is preferably 1% or less. More preferably, it is 0.5% or less, More preferably, it is 0.3% or less. The lower limit may be an impurity level. Generally about 0.001% is mixed on steelmaking.

Cr:
Since Cr has the effect of increasing hardenability and generating martensite and bainite in the ferrite structure, it may be added for these purposes. However, since the production | generation of a ferrite will be suppressed when it contains abundantly, it is preferable to make content 1% or less. The lower limit may be an impurity level. Generally about 0.02% is mixed in steelmaking.

Cu:
Since Cu has an action of precipitating at a low temperature to increase the strength, Cu may be added for the purpose of these actions. However, since there is a risk of causing grain boundary cracking of the slab, the content is preferably 3% or less. More preferably, it is 2% or less. In addition, when adding, it is preferable to make it content 0.1% or more. The lower limit may be an impurity level. Generally about 0.02% is mixed in steelmaking.

Ni:
Ni may be added for the purpose of increasing the stability of austenite at high temperatures. Further, when Cu is contained, it may be added in order to prevent grain boundary embrittlement of the slab. However, since the production | generation of a ferrite will be suppressed when it contains excessively, it is preferable to make content 1% or less. The lower limit may be an impurity level. Generally about 0.02% is mixed in steelmaking.

Mo:
Mo may be added to increase the strength by precipitating MoC, and because this precipitate suppresses the coarsening of ferrite and promotes the refinement of crystal grains. However, for the same reason as Ti and Nb, the ductility and workability are inhibited, so the content is preferably 1% or less. More preferably, it is 0.5% or less, More preferably, it is 0.3% or less. The lower limit may be an impurity level. Generally about 0.001% is mixed on steelmaking.

Ca, REM, B:
Ca, rare earth elements (REM), and B may be added alone or in combination to refine oxides and nitrides precipitated during solidification and maintain the soundness of the slab. However, since it is expensive, the total content is preferably 0.005% or less. The lower limit may be an impurity level.

  In addition, S, N, Sn etc. are mentioned as an "impurity" mixed in steel. About S and N, if possible, it is desirable to regulate the content thereof as follows.

S:
Since S is an impurity element that forms sulfide inclusions and degrades workability, the content is preferably suppressed to 0.05% or less. And when securing the further outstanding workability, it is preferable to set it as 0.008% or less. More preferably, it is 0.003% or less.

N:
N is an impurity element that lowers workability, and its content is preferably suppressed to 0.01% or less. More preferably, it is 0.006% or less.

(B) About the structure of the steel sheet according to the present invention The steel sheet according to the present invention is a steel sheet having a structure in which the main phase is ferrite and the main phase is a second phase other than ferrite. Here, the “main phase” means that the phase occupying the maximum proportion of the phase constituting the organization is the phase. The main phase ferrite is preferably at least 50% or more by volume, more preferably 60% or more. If the ferrite volume fraction is less than 50%, the ductility and workability of the steel sheet may be impaired.

  The crystal grain size (diameter) of ferrite greatly affects the mechanical properties and thermal stability of the steel sheet, and further the workability.

Therefore, in order to ensure sufficient strength, ductility, thermal stability and workability for the hot-rolled steel sheet according to the present invention, the average crystal grain size D of ferrite at a depth of 1/4 of the sheet thickness from the steel sheet surface. (Μm) must be kept within a certain range that satisfies the following formulas (1) and (2).
1.2 ≦ D ≦ 7 (1) Formula D ≦ 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² (2) Formula That is, the certain range is a range in which 1.2 μm is the lower limit and 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² μm and 7 μm is the upper limit. That is.

  And in order to ensure sufficient strength, ductility, thermal stability and workability for the cold-rolled steel sheet according to the present invention, the average crystal grain diameter D of ferrite at a depth of 1/4 of the sheet thickness from the steel sheet surface. (Μm) needs to be kept within a certain range that satisfies the following formulas (5) and (6).

1.2 ≦ D ≦ 9.3 (5) Formula D ≦ 5.0−2.0 · Cr + 5000 / (5 + 350 · C + 40 · Mn) ² ·· (6) That is, the fixed range is 1.2 μm as the lower limit, and 5.0−2.0 · Cr + 5000 / (5 + 350 · C + 40 · Mn) ² μm and 9 It is a range with the smaller value of 3 μm as the upper limit.

Here, the lower limit of the average grain size D of the ferrite is 1.2 μm. If the average grain size D is less than 1.2 μm, not only the work hardening coefficient is extremely reduced and the ductility and workability deteriorate, but also the fine ferrite structure This is because the thermal stability of the material deteriorates and the grains grow easily at a high temperature. In order to obtain more excellent ductility, workability and thermal stability, the lower limit of the average crystal grain size D of ferrite is preferably 1.5 μm. On the other hand, the upper limit of the average crystal grain size D of ferrite is 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² μm or 7 μm for hot-rolled steel plates, and 5.0− for cold-rolled steel plates. 2.0 · Cr + 5000 / (5 + 350 · C + 40 · Mn) The smaller value of ² μm and 9.3 μm, because if any of these values is exceeded, sufficient strength cannot be obtained. It is. In order to obtain better strength, the upper limit of the average grain size D of ferrite is set to the smaller value of 2.4 + 5000 / (5 + 350 · C + 40 · Mn) ² μm and 5.5 μm for hot-rolled steel sheets. For the cold-rolled steel sheet, it is preferable that the lower value of 4.5 + 5000 / (5 + 350 · C + 40 · Mn) ² μm and 8.5 μm be the upper limit. Here, a region surrounded by a large-angle grain boundary having a crystal orientation difference of 15 ° or more is defined as one crystal grain, and a small-angle grain boundary less than 15 ° is ignored.

In order to further improve the thermal stability of the steel sheet, it is preferable to keep the distribution of the crystal grain size of ferrite within a certain range. One factor that causes grain growth at high temperatures is the driving force based on the energy of the grain boundaries.If relatively large ferrite crystal grains are mixed in a fine ferrite structure, the large ferrite crystal grains As a driving force, it easily integrates with the surrounding fine ferrite crystal grains, and the grain growth proceeds rapidly. Therefore, to suppress the growth rate of ferrite crystal grains at high temperature, the ferrite crystal grains are refined and the average crystal grain size D (μm) satisfies the above formulas (1) and (2). In addition to staying within a certain range, the crystal grain size d (μm) occupies the crystal grains satisfying the following formula (4) among the ferrite at a depth of 1/4 of the plate thickness from the steel sheet surface. The area ratio is preferably 80% or more.
D / 3 ≦ d ≦ 3D (4) In other words, 80% or more of the ferrite crystal grains in terms of area ratio have an average crystal grain diameter D (μm). It is preferable that the particle size distribution be within a range of 1/3 to 3 times. Preferably, the grain size distribution is such that 85% or more of the ferrite crystal grains fall within a range of 1/3 to 3 times the average crystal grain diameter D (μm), more preferably 90% or more of the ferrite crystal grains. Is a particle size distribution that falls within a range of 1/3 to 3 times the average crystal grain size D (μm).

  The reason why the ferrite crystal grain size and its distribution are defined by a depth of 1/4 of the sheet thickness from the surface is that the ferrite crystal grain size of the hot-rolled steel sheet according to the present invention changes in the sheet thickness direction. The steel sheet according to the present invention can ensure desired mechanical properties and thermal stability by setting the ferrite crystal grain structure of this depth in the above range. In particular, the thermal stability of the particle size is not determined by the particle size distribution when taking statistics over a wide range from the surface of the plate to the inside, but by the particle size distribution when taking statistics at a specific depth. Determined. Therefore, if the structure is observed in a cross section parallel to the surface at a depth of 1/4 of the plate thickness, or if it is observed in a cross section perpendicular to the surface, it is within 100 μm from the depth of 1/4 of the plate thickness. Make observations and take statistics.

  The second phase other than ferrite may be a phase generally known to be produced in a low carbon steel material such as pearlite, cementite, bainite, martensite, retained austenite, carbonitride of elements other than Fe.

  In order to efficiently produce a steel plate with a yield ratio of 0.75 or more and excellent mechanical properties and thermal stability, as a second phase, bainite with a volume ratio of less than 50%, pearlite with less than 30%, less than 5% It is preferable to contain one or two or more selected from the group consisting of granular cementite, less than 5% martensite and less than 3% retained austenite in a total amount of less than 50%. More preferably, the total amount is less than 40%. When the volume fractions of bainite, pearlite, and granular cementite exceed the above values, workability is hindered. Moreover, when the volume ratio of martensite and retained austenite exceeds the above value, it becomes difficult to set the yield ratio to 0.75 or more.

  Next, in order to efficiently produce a steel sheet having a yield ratio of less than 0.75 and excellent mechanical properties and thermal stability, the second phase may contain 5-40% martensite by volume. preferable. In this case, it is preferable to reduce the volume ratio of bainite, pearlite, and granular cementite as much as possible. Residual austenite may be present, but in order to make it easier to lower the yield ratio, the volume ratio is preferably 3% or less.

  Further, in order to efficiently produce a steel sheet that is particularly excellent in elongation characteristics in which the product of the tensile strength TS and the total elongation El is 18000 or more and that is also excellent in thermal stability, residual austenite is used as the second phase in a volume ratio of 3 Include 30%. If the volume fraction of retained austenite is less than 3%, the elongation characteristics may be inhibited, and if it exceeds 30%, the thermal stability may be inhibited. The volume fraction of retained austenite contained as the second phase is preferably 5 to 25%.

  As the second phase other than ferrite, a trace amount of carbide, nitride, or oxide having a volume ratio of 1% or less can be contained in addition to the above-described one. These include Ti, Nb, V, Mo carbonitrides and the like.

(C) Grain growth rate at high temperature The temperature characteristics of the grain growth rate of a steel sheet in which the average crystal grain size of ferrite is within a certain range satisfying the above equations (1) and (2) is around 700 ° C. It is determined by the grain growth rate of ferrite at the following temperature. Therefore, the rate of increase X (μm / min) at 700 ° C. of the average crystal grain size of ferrite at a depth position of ¼ of the plate thickness from the steel sheet surface and the average crystal grain size D (μm) are the following (3) It is necessary to satisfy the equation.
D · X ≦ 0.1 ········································································································································ By maintaining the product D · X (μm ² / min) of the average grain size D (μm) at 0.1 μm ² / min or less, it becomes stable against the main thermal history in the welding or hot dipping process, Good thermal stability is obtained. In order to obtain better thermal stability, the product D · X is preferably 0.07 μm ² / min or less, and more preferably 0.05 μm ² / min or less.

In addition, as shown in Examples 2 and 3 to be described later, the product D · X (μm ² / min) of the increase rate X (μm / min) of the average crystal grain size of ferrite and the average crystal grain size D (μm) However, the ferrite crystal grain structure of the steel sheet of 0.1 μm ² / min or less shows almost no change in grain size even when heat-treated at 850 ° C. for several tens of seconds. Unlike normal grain growth, which is proportional to the square root of time, the crystal grain size (diameter) of ferrite in the steel sheet according to the present invention increases at approximately 700 ° C. in proportion to time. Therefore, the increase rate X (μm / min) of the average crystal grain size of ferrite is obtained by measuring the grain size change at about 700 ° C. for about 1 hour and averaging the rate of change.

In order to further reduce the grain growth rate, the dislocation density in the ferrite crystal grains is preferably 10 ⁹ / cm ² or less, more preferably 10 ⁸ / cm ² or less.

(D) About Zn plating Fine grain hot rolled steel sheet having the above-mentioned structure and its thermal stability is coated with Zn, Zn-Al alloy, Al-Si alloy, Fe-Zn alloy, etc. using a hot dipping line. Can be applied to the steel sheet surface.

As a composition of the plating bath of the Zn—Al alloy, a Zn— (0.1-60)% Al bath, a bath in which Si and / or Mg are added in combination, and the like are used. Moreover, as a composition of the plating bath of the Al—Si alloy, an Al— (7-13)% Si bath or the like is used. There is no particular problem even if the plating bath contains 0.1% or less of Fe, V, Mn, Ti, Nb, Ca, Cr, Ni, W, Cu, Pb, Sn, Cd, and Sb. The composition of the film on the surface of the steel sheet cooled after the plating generally has a slightly higher Fe concentration than the plating bath composition, because interdiffusion of elements occurs between the steel material and the molten metal during immersion and cooling. The alloyed hot dip galvanizing positively utilizes this mutual diffusion, and the Fe concentration in the film is 7 to 15%. Although the amount of plating is not particularly limited, it is preferably 30 to 200 g / m ² per side, and in the case of alloyed hot dip galvanizing, there is a concern about powdering, so 25 to 60 g / m. ² is preferable.

  The plating method using the hot dipping line is as follows.

The hot-rolled steel sheet that has achieved a fine grain structure is passed through a continuous hot-dip galvanizing line after removing the surface scale through the pickling process. From the entry side, after alkali degreasing and washing with water, after preheating, it is heated to a temperature of 550 to 900 ° C. in an atmosphere containing hydrogen, and the Fe oxide on the steel sheet surface is reduced, which is suitable for the subsequent plating treatment. Forming a surface. When the temperature is lower than 550 ° C., the reduction is not sufficient, and when heated to a temperature exceeding 900 ° C., the ferrite structure becomes coarse. In order to obtain a ferrite + pearlite structure or a ferrite + cementite structure after plating, the temperature is preferably from 550 ° C. to around 730 ° C. On the other hand, bainite as a second phase, martensite, to thereby produce a residual γ, etc., who are heated from _point A to a two-phase coexisting temperature region of ferrite and austenite to 900 ° C. are preferred. The hydrogen content in the atmosphere is preferably 5 to 40%. If the hydrogen content is less than 5%, the reduction is not sufficiently performed. If it exceeds 40%, the cost of the atmospheric gas increases excessively. Components other than hydrogen may be any gas that does not inhibit reduction. Nitrogen is preferred from the viewpoint of cost. The soaking time is not particularly specified as long as the reduction is sufficiently performed, and is generally 10 seconds or longer. The upper limit is 5 minutes or less, more preferably 2 minutes or less in order not to coarsen the ferrite. After passing through heating and soaking for this reduction, the steel sheet temperature is cooled to the vicinity of the plating bath temperature, adjusted to a predetermined adhesion amount after being immersed in the plating bath, and cooled to room temperature. In the case of alloying hot dip galvanizing, after hot dip galvanizing as described above, reheating to 470 to 600 ° C. causes a reaction between the base iron and the plating film, and the Fe—Zn alloy film is formed on the steel sheet surface. Form.

  Thus, in the hot dipping method, the steel sheet is not only heated in the plating bath, but also in the process of reducing the surface oxide layer before being immersed in the plating bath, and in the alloying step after immersion in the plating bath, high-temperature heat treatment. Receive. However, since the ferrite structure of the steel sheet of the present invention is thermally stable, the fine grain structure is maintained even after these steps and exhibits excellent mechanical properties. Further, since the ferrite crystal grains on the surface are fine, the alloying reaction rate is increased, and there is an advantage that the production can be efficiently performed.

  In addition, as steel composition in the case of plating, C: 0.001 to 0.15%, Si: 0.005 to 1.5% and / or P: 0.005 to 1.0% Is preferred.

(E) About weldability In the steel plate which has the fine grain structure created by the conventional low temperature rolling, since it is inferior to thermal stability and a HAZ part softens, the characteristic of a welded part will fall. On the other hand, the thermal stability of the steel sheet according to the present invention is good even when the steel sheet itself or the steel sheet with the above-mentioned surface coating is joined by welding, and welding such as laser, spot, arc, etc. The formability of the welded part after welding is improved.

In fusion welding represented by arc / plasma welding and laser welding, the carbon equivalent Ceq (I) defined by the following formula (7) is 0.06 to 0.6% with respect to the chemical composition of the steel sheet. It is preferable to specify.
Ceq (I) = C + Mn / 6 + Si / 24 + Cr / 5
+ Mo / 4 + Ni / 40 + V / 14 (7) Formula Here, the element symbol in the formula indicates the content (unit: mass%) of each element in the steel.

  Ceq (I) is an index of the maximum hardness of the welded portion, and by defining Ceq (I) as 0.06 to 0.6%, the formability of the member after welding can be ensured. When Ceq (I) is less than 0.06%, the hardenability is poor, and therefore the hardness of the weld metal part is softer than the hardness of the base material reinforced by the thermally stable fine grain structure. The workability of is reduced. If the content exceeds 0.6%, the weld metal part and the HAZ part having thermal stability are hardened by quench hardening, and the formability of the weld part is deteriorated because the hardening to the base metal hardness is remarkable. Ceq (I) is preferably specified to be 0.10 to 0.5%. Moreover, it is preferable that the C content that causes hardening and embrittlement of the welded portion is 0.17% by mass or less.

On the other hand, in spot welding in which resistance welding is performed by energization heat generation to the base material, in order to ensure joint strength, the C content is set to 0.17 with respect to the chemical composition from the viewpoint of hardness distribution of the weld and suppression of embrittlement. The carbon equivalent Ceq (II) defined by the following formula (8) is specified as 0.03 to 0.20%, and the nugget diameter for securing the joint strength is specified. In order to obtain in a wide range of conditions, it is preferable that the base material resistance index Rsp defined by the following formula (9) is specified to be 45 or less.
Ceq (II) = C + Mn / 100 + Si / 90 + Cr / 100 (8) Formula Rsp = 13.5 × (Si + Al + 0.4Mn + 0.4Cr) +12.2 (9) where the element symbol in the formula Indicates the content of each element in steel (unit: mass%).

  In a rapid thermal cycle such as spot welding, since the effect of C content on hardening and embrittlement is large, the C content is preferably 0.17% or less, more preferably 0.15% or less. .

  Ceq (II) is an index of the maximum hardness of a welded part in a rapid thermal cycle such as spot welding. By specifying Ceq (II) as 0.03 to 0.20%, resistance weldability is excellent. A welded part can be obtained. When Ceq (II) is less than 0.03%, the hardenability is poor, so the maximum hardness obtained at the spot welded portion is smaller than the base metal hardness, and so a so-called button fracture is obtained in the joint strength evaluation test. However, the maximum breaking load that can be obtained is reduced. When Ceq (II) exceeds 0.20%, hardening and embrittlement with respect to the base metal hardness is remarkable in the weld metal part and the HAZ part having thermal stability due to quench hardening. The molten metal part (in the nugget) is cracked, making it difficult to obtain a so-called button break. In addition, it is preferable to prescribe | regulate Ceq (II) as 0.06 to 0.17%.

  The base material resistance index Rsp is an index of how wide the welding condition range can provide a sufficient nugget (molten joint) diameter to ensure joint strength, and is excellent in resistance weldability. In order to obtain a welded portion, it is preferable to make it 45 or less. More preferably, it is 40 or less.

  In order to obtain a nugget diameter for securing joint strength in a wide range of welding conditions, current density and resistance heating are important. Here, the current density is determined by the cross-sectional area of the energization path during welding, and the steel of the present invention, which is excellent in thermal stability, does not cause softening due to grain growth, thus suppressing the initial energization path spread, Sufficient nugget diameter is easily formed. On the other hand, the resistance heat generation is greatly affected by the electric resistance value of the base material. When the base material resistance is large, excessive resistance heat generation occurs, and dust generation is likely to occur when the appropriate condition range is exceeded. In general, the appropriate condition range for spot welding is a range of welding current to dust generation current (where t is the plate thickness of the joining material) in which a nugget diameter of 4 × √t is formed, or a minimum current indicating button fracture. ~ Expressed using the range of dust generation current.

(F) About rolling Rolling is performed in the austenite temperature range from a temperature exceeding 1000 ° C. using a lever mill or a tandem mill. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last several stages.

Use slabs obtained by continuous casting or casting / bundling, steel plates obtained by strip casting, etc., and if necessary, those once hot or cold worked, and if they are cold pieces, 1000 ° C Reheated to a temperature exceeding 30 ° C and rolled. When the rolling start temperature is 1000 ° C. or less, the rolling load becomes excessive and it becomes difficult to obtain a sufficient rolling rate, and rolling at a sufficient rolling rate may be terminated at a temperature of ₃ or more points at Ar. This makes it difficult to obtain desired mechanical properties and thermal stability. Rolling is preferably started at a temperature of 1025 ° C. or higher, more preferably 1050 ° C. or higher. The upper limit is set to 1350 ° C. or lower, preferably 1250 ° C. or lower in order to suppress coarsening of austenite grains and to suppress equipment costs and heating fuel costs. In the case of a steel type in which it is not necessary to sufficiently dissolve precipitates such as TiC and NbC in austenite, it is preferable to reheat to a relatively low temperature (1050 to 1150 ° C.) within this range. This is because the initial austenite crystal grains are refined and the final ferrite crystal grains are easily refined.

The rolling finishing temperature is set to a temperature range of Ar ₃ points or more and 780 ° C. or more in order to transform from austenite to ferrite after rolling. When the finishing temperature is lower than Ar ₃ point, ferrite is generated during rolling. If the temperature is lower than 780 ° C., the rolling load increases and it becomes difficult to apply sufficient reduction, and ferrite transformation may occur in the surface layer portion during rolling. Preferably, the rolling is finished at a temperature of Ar ₃ points or higher and 800 ° C. or higher.

The temperature to terminate the rolling, the better low if the temperature range of more than Ar ₃ point and 780 ° C.. This is because the effect of accumulating processing strain introduced into austenite by rolling increases, and the refinement of crystal grains is promoted. The Ar ₃ point of the steel type used in the present invention is approximately 780 to 900 ° C.

  The total reduction amount is 90% or more, preferably 92%, more preferably 94% or more in terms of sheet thickness reduction rate in order to promote the refinement of ferrite. The sheet thickness reduction rate in the temperature range from the rolling end temperature to “rolling end temperature + 100 ° C.” is preferably 40% or more. More preferably, the sheet thickness reduction rate in the temperature range from the rolling end temperature to “rolling end temperature + 80 ° C.” is 60% or more. The rolling is continuous multi-pass rolling. The amount of reduction per pass is preferably 15 to 60%. A larger rolling reduction per pass is preferable from the viewpoint of accumulating strain into austenite and refining the crystal grain size of ferrite produced by transformation, but it requires an increase in rolling load. Not only increases in size but also makes it difficult to control the shape of the plate. In the method of the present invention, fine ferrite crystal grains can be obtained even by rolling in a plurality of passes with a reduction amount per pass of 40% or less. Therefore, in particular, when it is desired to easily control the plate shape, it is preferable that the rolling reduction rate of the final two passes is 40% / pass or less.

(G) Cooling after rolling In order to generate a fine ferrite grain structure by transforming from austenite to ferrite as a driving force without releasing the processing strain introduced into austenite after rolling is finished. Then, it is cooled to a temperature of 720 ° C. or less within 0.4 seconds from the end of rolling. Preferably, it is cooled to a temperature of 720 ° C. or less within 0.2 seconds from the end of rolling. It is desirable to use water cooling for cooling, and the cooling rate is preferably 400 ° C./second or more as an average cooling rate during the period of forced cooling excluding the air cooling period.

  Here, the reason for prescribing the time until cooling to a temperature of 720 ° C. or lower was introduced by processing before fine ferrite was formed when cooling was stopped or slowed at a temperature exceeding 720 ° C. This is because the strain is released or the existence form of the strain is changed, so that it becomes ineffective for nucleation of ferrite and the ferrite crystal grains are remarkably coarsened.

  When the temperature reaches 720 ° C. or lower, it enters a transformation temperature range in which ferrite transformation is activated. The ferrite transformation temperature range where the above ferrite structure is obtained is a temperature range between this temperature and 600 ° C. Therefore, after reaching 720 ° C. or lower, the cooling is temporarily stopped, or the speed thereof is slowed down and held at this temperature range for 2 seconds or more, so that the formation of the above thermally stable ferrite crystal grain structure is ensured. Can be. If the holding time in this temperature range is short, the formation of the thermally stable ferrite crystal grain structure may be hindered. More preferably, it is good to make it stay for 3 seconds or more in the temperature range of 620-700 degreeC.

  In the case of a multi-phase structure steel having a fine ferrite crystal grain structure as a main phase and martensite of 5% or more in volume ratio dispersed therein, after cooling and holding as described above, the temperature reaches 350 ° C. or less. It is preferable to cool. It is more preferable to cool to a temperature of 250 ° C. or lower at a cooling rate of 40 ° C./s or higher. In addition, when cooling to a temperature of 350 ° C. or less is performed at a cooling rate of 20 ° C./s or less, bainite is likely to be generated, and martensite formation may be hindered.

  On the other hand, in the case of a dual phase structure steel mainly composed of a fine ferrite crystal grain structure and 3 to 30% of retained austenite is dispersed in volume ratio, after cooling as described above, the cooling rate is set to 350 ° C. at a cooling rate of 20 ° C./s or more. It is preferable to cool to ˜500 ° C. and then gradually cool at a cooling rate of 60 ° C./h or less. The cooling rate from 400 to 500 ° C. is more preferably 50 ° C./s or more.

(H) About cooling equipment In this invention, the equipment which performs said cooling is not limited. Industrially, it is preferable to use a water spray device having a high water density. For example, a water spray header can be arrange | positioned between rolling plate conveyance rollers, and it can cool by injecting high-pressure water with sufficient water quantity density from the upper and lower sides of a plate.

(I) Cold rolling and annealing In order to efficiently produce a thin steel sheet having a fine grain structure, it is hot-rolled, pickled, further cold-rolled, and then annealed. The cold rolling rate is set to 40% or more to promote recrystallization of ferrite during annealing, and is set to 90% or less because rolling becomes difficult. There is no restriction | limiting in rolling equipment, A tandem mill and a reverse mill can be used.

After cold rolling, heat treatment is performed in order to recrystallize the processed ferrite to obtain a fine-grained ferrite structure. The temperature is set to 900 ° C. or lower in order to prevent coarsening of crystal grains at a temperature higher than the temperature at which ferrite recrystallization occurs. Preferably, the temperature is at least _one Ac and at most _three Ac. If the Ac is less than ₁ point, it takes a long time to recrystallize the ferrite, and if the Ac exceeds ₃ points, the structure becomes an austenite single phase and the structure is likely to be coarsened. The annealing time is longer than the time for ferrite to recrystallize, and there is no upper limit. Although normal continuous annealing equipment, batch annealing equipment, etc. can be used, it is preferable to perform annealing for a short time using continuous annealing equipment for efficient production. When performing hot dipping using a continuous hot dipping equipment, there is generally no pre-annealing process in the plating equipment, so there is no need to anneal after cold rolling, and the cold rolled material is passed directly through the plating equipment. can do.

  Hereinafter, the present invention will be described in more detail by way of examples.

  Steels of steel types A1 to A11 having chemical compositions shown in Table 1 were melted and made 30 mm thick by hot forging. Then, after reheating to 1050 degreeC or more, it rolled by the small tandem mill for a test, and finished to 2 mm of plate | board thickness.

Table 2 shows the rolling finishing temperature and cooling conditions. In all rolling, the finishing temperature of rolling was higher than the Ar ₃ point of each steel type, and further, multi-pass rolling of 3 passes or more was performed within the temperature range of finishing temperature to [finishing temperature + 100 ° C.]. The final two-pass rolling was light rolling at 35% / pass or less except for test number 3. For test number 3, the final two passes were 50% to 60% large rolling. After the rolling finish, as shown in Table 2, it was cooled to a predetermined temperature within a temperature range of 500 to 720 ° C. by water cooling. Depending on the test number, a holding time at 720 to 600 ° C. was provided by providing an air cooling time after water cooling. In Table 2, in addition to the holding time in the temperature range of 720 to 600 ° C., the holding time in the temperature range of 700 to 620 ° C. is also shown. Thereafter, water cooling to room temperature is performed at a rate of about 100 ° C./s, or by performing furnace cooling in the furnace after water cooling to a predetermined temperature within a temperature range of 600 to 400 ° C. A steel sheet having the following structure was prepared.

  About the structure | tissue of the hot-rolled steel plate obtained in this way, the cross section of the steel plate thickness was observed by using a scanning electron microscope.

  The crystal grain size and grain size distribution of ferrite were determined by conducting crystal orientation analysis using the EBSP (Electron Back Scattering Pattern) method at a depth of ¼ of the plate thickness from the plate surface. The volume ratio of each phase was measured by observing a structure corroded with nital or picric acid at a depth of 1/4 of the plate thickness from the plate surface using a scanning electron microscope. The structure of the second phase other than the ferrite phase of the steel sheet produced in this example was pearlite, bainite, and intragranular spherical cementite or grain boundary cementite.

  Regarding the steel plate of the present invention, the crystal grain size at a depth of 100 μm from the steel plate surface and the crystal grain size at the center of the plate thickness were measured by the same method as described above. As a result, the crystal grain size at a depth of 100 μm is 60% or less of the grain size at the center of the plate thickness, and the grain size at a depth of ¼ of the plate thickness is the grain size at the center of the plate thickness. It was 85% or less.

  For mechanical properties, tensile properties were measured with JIS No. 5 tensile test pieces, and tensile strength TS (MPa), yield ratio YR and total elongation El (%) were evaluated.

For thermal stability, after immersing in a salt bath at 700 ° C. for 10, 30 or 60 minutes, quench, measure the particle size by the same method as described above, and determine the pre-annealing particle size d ₀ (μm) and annealing. An increase rate X (μm / min) of the average crystal grain size was calculated by dividing the difference of the post grain size d ₁ (μm) by the annealing time (min).

  Table 3 shows the structure, properties, and tensile test results of the hot-rolled steel sheet thus obtained. Here, since test number 1 has a short holding time in the temperature range of 720 to 600 ° C. of 0.8 seconds, the ferrite volume fraction is not only as small as 14.8%, but also the grain growth when annealed at 700 ° C. High speed and poor thermal stability. Test No. 3 employs large rolling at low temperature, so the particle size is excessively fine, 1.13 μm, and is inferior in thermal stability and strength / elongation balance. Test No. 5 took 1.52 seconds to cool down to 720 ° C. immediately after finishing rolling, so that the average crystal grain size of ferrite became 4.52 μm and coarsened and became a mixed grain structure Inferior in thermal stability. Test No. 19 has an extremely short holding time of 0.2 seconds in a temperature range of 720 to 600 ° C., and therefore has a bainite structure exceeding 95% and a ferrite volume fraction of less than 5%. In contrast to these comparative examples, the inventive examples in which the cooling conditions are within the scope of the present invention are excellent in both thermal stability and mechanical properties.

A steel piece (size: 80 mm width × 100 mm length × 35 mm thickness) having a chemical composition shown in Table 4 is hot-rolled at a temperature of ₃ or more Ar under the conditions shown in Table 5, and then water-cooled. A hot rolled steel sheet having a thickness of 1.2 mm was obtained.

The obtained hot-rolled steel sheet was measured for ferrite average crystal grain size, its grain size distribution and dislocation density at a depth of ¼ of the plate thickness from the plate surface, and thermal stability was evaluated. . The ferrite crystal grain size, the grain size distribution and the dislocation density were measured, and the thermal stability was evaluated by the same method as described above. The dislocation density ρ (cm ⁻² ) was determined by measuring the number L of intersection points between the length L (cm) of an arbitrary line segment and the dislocation line in a bright field image by observation with a transmission electron microscope, and measuring the film thickness t ( cm) was obtained according to the following equation (10).
ρ = 2N / Lt (10) Formula Regarding thermal stability of ferrite grains, after soaking in a salt bath at 700 ° C. for 10, 30 or 60 minutes Quickly cool, measure the particle size in the same way as described above, and divide the difference between the pre-annealed particle size d ₀ (μm) and the post-annealed particle size d ₁ (μm) by the annealing time (min). From the above, the increase rate X (μm / min) of the average crystal grain size was calculated.

  Regarding the steel sheet of the present invention, the crystal grain size at a depth of 100 μm from the steel sheet surface and the crystal grain size at the center part of the plate thickness were measured by the same method as described above. As a result, the crystal grain size at a depth of 100 μm is 60% or less of the grain size at the center of the plate thickness, and the grain size at a depth of ¼ of the plate thickness is the grain size at the center of the plate thickness. It was 85% or less.

  In order to clarify the influence of the heat treatment on the mechanical properties of the hot-rolled steel sheet thus obtained, the reheat treatment was performed in the range of 730 to 830 ° C., and then the ferrite average crystal grain size was measured again. . Here, as for the mechanical properties, tensile properties were measured with a JIS No. 5 tensile test piece, and the tensile strength TS (MPa), the yield ratio YR, and the total elongation El (%) were evaluated.

  Table 6 shows the structure, properties and tensile test results of the hot-rolled steel sheet thus obtained, and after reheating treatment in the range of 730 to 830 ° C., the average ferrite grain size was measured again. The results are shown. Here, in test numbers C and F, the steel sheet after hot rolling is inferior in mechanical properties and thermal stability. Then, it was confirmed that the reheat treatment increased the ferrite crystal grain size to more than 8 μm and further deteriorated the mechanical properties. In contrast to these comparative examples, the steel sheet having excellent thermal stability according to the present invention exhibits excellent mechanical properties and has a particle size of almost no grain even after heat treatment at 730 ° C. to 830 ° C. for several tens of seconds. No change is shown. Therefore, it was confirmed that the steel sheet according to the present invention was strengthened with fine grains even after the heat treatment.

Steels of steel types AA to AZ having chemical compositions shown in Table 7 were melted and made 30 mm thick by hot forging. Then, after reheating to a temperature of 1100 to 1200 ° C., rolling was performed for 5 passes at a temperature higher than the Ar ₃ point to finish a plate thickness of 2 mm. The final two-pass rolling was light rolling at 35% / pass or less. After rolling, it was cooled under the conditions shown in Table 8. The structure of the obtained steel material observed the cross section of the steel plate thickness using the scanning electron microscope (SEM).

  The ferrite crystal grain size and grain size distribution were determined by conducting crystal orientation analysis using the EBSP (Electron Back Scattering Pattern) method at a depth of 1/4 of the plate thickness from the plate surface. The volume ratio of each phase was measured by observing the structure corroded with nital or picric acid at a depth of ¼ of the plate thickness from the plate surface using SEM. The ferrite volume fraction and martensite volume fraction were measured by a so-called mesh method at a depth of ¼ of the plate thickness from the plate surface, and indicated by their arithmetic average values. Furthermore, a JIS No. 5 test piece was collected from the rolled material, and the mechanical properties were evaluated by a normal temperature tensile test.

  Regarding the steel sheet of the present invention, the crystal grain size at a depth of 100 μm from the steel sheet surface and the crystal grain size at the center part of the plate thickness were measured by the same method as described above. As a result, the crystal grain size at a depth of 100 μm is 60% or less of the grain size at the center of the plate thickness, and the grain size at a depth of ¼ of the plate thickness is the grain size at the center of the plate thickness. It was 85% or less.

  For thermal stability, the sample was immersed in a 700 ° C. salt bath for 10, 30 or 60 minutes, then rapidly cooled, and the average crystal grain size increase rate X (μm / min) was calculated in the same manner as described above. .

These results are shown in Table 9. Here, the test number A10 has a ferrite crystal grain size as coarse as 4.57 μm and the volume fraction of ferrite is small, and the test numbers A8 to A9 and A11 have an average crystal grain size increase rate X and an average Since the product D · X of the crystal grain size D exceeds 0.1 μm ² / min and the volume fraction of ferrite is small, the mechanical properties are inferior and the thermal stability is inferior. In contrast to these comparative examples, the present invention examples of test numbers A1 to A7 have a fine ferrite crystal grain size of about 2.5 μm despite the fact that the rolling is performed under light rolling, and 50% by volume or more of ferrite. The steel sheet is made of 10% by volume or more of martensite. These ferrite structures are thermally stable and have a ferrite structure containing an appropriate amount of martensite, so that high strength and good elongation characteristics can be obtained.

Steels of steel types A1 to A10 having chemical compositions shown in Table 10 are melted and 35 by hot forging.
The thickness was set to mm. Then, after reheating to a temperature of 1050 to 1250 ° C., subjected to rolling five passes at a temperature higher than the Ar ₃ point, it was finished to a thickness of 1.5 mm. After rolling, it was cooled under the conditions shown in Table 11. The structure of the obtained steel material observed the cross section of the steel plate thickness using the scanning electron microscope (SEM).

  The ferrite crystal grain size and grain size distribution were determined by conducting crystal orientation analysis using the EBSP (Electron Back Scattering Pattern) method at a depth of 1/4 of the plate thickness from the plate surface. The volume ratio of each phase was measured by observing the structure corroded with nital or picric acid at a depth of ¼ of the plate thickness from the plate surface using SEM. The ferrite volume fraction was measured by a so-called mesh method at a depth of ¼ of the plate thickness from the plate surface, and indicated by an arithmetic average value thereof. The residual austenite volume fraction was determined from X-ray diffraction measurement. Furthermore, a JIS No. 5 test piece was collected from the rolled material, and the mechanical properties were evaluated by a normal temperature tensile test. For thermal stability, the sample was immersed in a 700 ° C. salt bath for 10, 30 or 60 minutes, then rapidly cooled, and the average crystal grain size increase rate X (μm / min) was calculated in the same manner as described above. .

  Regarding the steel sheet of the present invention, the crystal grain size at a depth of 100 μm from the steel sheet surface and the crystal grain size at the center part of the plate thickness were measured by the same method as described above. As a result, the crystal grain size at a depth of 100 μm is 60% or less of the grain size at the center of the plate thickness, and the grain size at a depth of ¼ of the plate thickness is the grain size at the center of the plate thickness. It was 85% or less.

  These results are shown in Table 12. Here, in the inventive examples of Test Nos. 1 to 8 and 10, ferrite is fine and excellent in thermal stability and mechanical properties. On the other hand, the comparative examples of test numbers 9 and 11 to 13 are inferior in thermal stability and mechanical properties as compared with the inventive examples.

  After hot-rolling a 50 mm thick slab made of steel of steel types A to C having the chemical composition shown in Table 13 with 6 consecutive passes under the rolling conditions shown in Table 14 and having a total reduction of 96%, Table 14 shows The steel sheet was cooled under the cooling conditions shown to obtain a steel sheet having a thickness of 2 mm.

  Samples 1, 4 and 6 of the present invention and samples 3 and 5 of the comparative example were subjected to a rolling reduction of 40 to 35% in the final stage 2 passes, and samples 2 and 7 of the comparative example were subjected to the final pass by low temperature rolling. The rolling rate was a large reduction of 65%. After rolling, pickling treatment was performed to remove the scale. This specimen was cut into a size of 80 × 200 mm, and plated using a vertical hot-dip Zn plating apparatus under the following conditions.

First, a steel plate having a thickness of 2.0 mm was degreased and washed with a NaOH solution at 75 ° C. and annealed at 600, 720, or 840 ° C. for 60 seconds in an atmosphere of N ₂ + 20% H _{2 and a} dew point of −40 ° C. After annealing, the steel sheet was cooled to near the bath temperature, immersed in various plating baths for 3 seconds, and then the amount of coating on one side of the plating was adjusted to 50 g / m ² by a wiping method. When the alloying treatment was performed, the plated steel plate was subsequently subjected to a heat treatment at 500 ° C. for 30 seconds using an infrared heating device. The cooling rate was adjusted by changing the air volume and mist volume. In the temper rolling after plating, Ra of the roll was set to 1 to 5 μm, and the load was set to 200 ton / m.

  EBSP shows the ferrite grain size and grain size distribution at 1/4 of the plate thickness from the surface of the steel sheet after hot rolling and plating, and the volume fraction of each phase of the steel structure is observed by SEM observation of the corrosion structure and X-ray diffraction By the measurement, the dislocation density in the ferrite crystal grains was further investigated with a transmission electron microscope. Moreover, about the steel plate after plating, JIS No. 5 tensile test piece was extract | collected and the tensile characteristic was investigated.

  Regarding the steel sheet of the present invention, the crystal grain size at a depth of 100 μm from the steel sheet surface and the crystal grain size at the center part of the plate thickness were measured by the same method as described above. As a result, the crystal grain size at a depth of 100 μm is 60% or less of the grain size at the center of the plate thickness, and the grain size at a depth of ¼ of the plate thickness is the grain size at the center of the plate thickness. It was 85% or less.

  These results are shown in Table 15. Here, since the hot-rolled steel sheets having fine ferrite crystal grain structures according to the inventive examples of test numbers 1 to 3, 6 to 8, 10 and 12 have high thermal structure stability, they can be subjected to hot dipping treatment. The ferrite crystal grain size hardly increases, and the fine ferrite structure maintains an appropriate grain size distribution and low dislocation density. Therefore, both mechanical properties and thermal stability are excellent even after plating. On the other hand, the comparative examples of test numbers 4 to 5, 9 and 11 are inferior in thermal stability and mechanical properties as compared with the inventive examples.

  Plasma welding (welding speed: 0.5 m / min, welding current: about 180 A) and laser welding (welding speed: 1.0 m / min, condensing) to the steel sheet having a thickness of 2 mm shown in Examples 1, 3 and 5. Butt through welding was performed with a spot diameter of 0.6 mm and an output of 3000 W. Table 16 shows the main components and carbon equivalent Ceq (I) of the steel sheet.

Characteristic evaluation of the obtained welded part was carried out using a ball head overhang test having a diameter of 50 mm. The shape of the ball head projecting specimen is shown in FIG. (a) is when the principal strain direction is parallel to the direction of the weld line 2 (Type I), and (b) is when the principal strain direction is perpendicular to the direction of the weld line 2 (Type I).
II). The ball head overhang test specimen 1 was cut out from each welded portion, and the overhang height and breakage position were evaluated.

The results are shown in Table 17. Here, the test number 13 has a small Ceq (I), and the bead portion melted and solidified during welding is softer than the base metal, and the bead breaks in the Type II test. In the case of test number 14, since Ceq (I) is excessive, the bead portion is excessively cured, and a bead crack occurs in the Type I test. Test No. 15 is a fine-grained steel sheet produced by low temperature rolling, which is inferior in thermal stability, so that the HAZ part softens despite the bead part having an appropriate hardness.
Fracture occurs in HAZ in the test of II. On the other hand, Test Nos. 1 to 12 show high workability at the site including the welded portion and are excellent in formability after welding even when fusion welding using plasma or laser is performed.

  A fine-grain hot-rolled steel sheet with a thickness of 2 mm having a tensile strength TS of 440 to 780 MPa class as shown in Examples 1, 3, 4 and 5, and a commercially available coarse-grain hot-rolled steel sheet having substantially the same tensile strength as these. Used to evaluate resistance weldability. Table 18 shows the tensile strength TS, main chemical composition, carbon equivalent Ceq (II), and the like of each hot-rolled steel sheet.

  A test piece of 30 × 100 mm was cut out, overlapped with a lap allowance of 30 mm, and various welding currents were applied using an 8 mm diameter dome-shaped electrode with a pressure of 3920 N (400 kg · f) and an energization time of 30 cycles. The joint was made by changing to.

  While measuring the generation current of dust, a shear tensile test was performed to evaluate the maximum breaking load of the joint. In addition, the cross-sectional macro observation of spot welds is performed, and the nugget diameter after picric acid corrosion is measured, so that the current range from 4√t nugget diameter formation to dust generation and the current from button breakage to dust generation of the joint. A range was determined to evaluate resistance weldability. Note that the maximum breaking load of the joint indicates the maximum breaking load in the joint at which the minimum button diameter was obtained among the conditions in which the button was broken.

  The results are shown in Table 19. Here, in Test No. 1, since Ceq (II) is too small, the maximum hardness of the welded portion is low, and the joint strength is reduced. In Test Nos. 10 to 12, since Ceq (II) or Rsp is excessive, the current range from button breakage to dust generation is small. Test No. 14 is a fine-grained steel plate produced by low-temperature rolling, which is inferior in thermal stability. Therefore, although the bead portion has an appropriate hardness, the HAZ portion is softened, so the joint strength is low. Test numbers 3, 6 and 13 are commercially available coarse-grain hot-rolled steel sheets, which are inferior in thermal stability and all have a small current range from button breakage to dust generation. On the other hand, the steel plates with test numbers 2, 4 to 5 and 7 to 9 have excellent mechanical properties, a wide welding appropriate current range, and excellent resistance weldability.

A steel having a chemical composition A1 shown in Table 1 of Example 1 was melted and made 30 mm thick by hot forging. Then, after reheating to 1000 degreeC, it rolled by the small tandem mill for a test, and was finished to 1.3 mm in plate | board thickness. The rolling finishing temperature was 830 ° C., which was higher than the Ar ₃ point. The final three-pass rolling was 40-50% large reduction rolling. After rolling finish, water cooling is started after 0.05 seconds, cooled to a temperature of 680 ° C. at a cooling rate of 1000 ° C./second or more, allowed to cool for about 4 seconds, reaches 600 ° C., and then water cooled again. And cooled to room temperature. Thereafter, pickling, cold rolling to a thickness of 0.5 mm at a reduction ratio of 62%, and annealing were performed. The annealing was performed by immersing in a salt bath at 800 ° C. for about 2 minutes and then cooling to room temperature.

The average crystal grain size of the ferrite after hot rolling was 1.4, 1.7, and 2.3 μm at the 1/16 depth, 1/4 depth, and the center position of the plate thickness from the surface, respectively. It was. The volume fraction of ferrite was 93%, and the second phase was bainite or martensite. Further, at a depth position of 1/4 of the plate thickness from the surface, 90% or more of the ferrite has a grain size between 1/3 to 3 times the average grain size of the depth, and the grain growth rate at 700 ° C. And the particle size product X was 0.026 μm ^{2 /} min (= 0.015 μm / min × 1.7 μm). FIG. 2 shows the time change of the grain size of ferrite at 700 ° C., and it can be seen that the grain growth rate is as small as 0.015 μm / min. The form of the ferrite grains is equiaxed.

FIG. 3 shows the change of the grain size after cold rolling and annealing depending on the annealing time. The data with no annealing time is the grain size after hot rolling. Although the grain size is increased by annealing, the change is small, and after annealing, 1/16 depth, 1/4 depth, and 2.2, 2.4, and 2. It is as fine as 9 μm. There is no coarsening due to the extended annealing time. The crystal grain size of the commercial cold-rolled steel sheet having the same composition is about 7.5 μm, and the grain size of the steel sheet of the present invention is about 1/3. FIG. 4 shows an example of the structure at this time (800 ° C., 5 min annealing). The volume fraction of ferrite after annealing of the steel sheet of the present invention was about 70%, and the second phase was martensite. The product D · X of the grain growth rate and the particle diameter at 700 ° C. was about 0.02 μm ² / min. Further, at a depth of 1/4 of the plate thickness from the surface, 90% or more of the ferrite had a particle size between 1/3 and 3 times the average particle size of the depth.

  Similarly, the hot-rolled steel sheet of Example 8 was pickled, cold-rolled at a reduction ratio of 62% until a thickness of 0.5 mm was obtained, and then annealed. The annealing simulated the heat treatment process of an industrial continuous annealing line. The heating rate is 10 to 15 ° C./second, the soaking temperature is 750 ° C. or 800 ° C., and the cooling condition after rolling is equivalent to continuous Zn alloy plating when the soaking temperature is 750 ° C. When the heat temperature was 800 ° C., an overaging treatment for gradually cooling from 400 ° C. to 320 ° C. was added.

When the soaking temperature is 750 ° C., the ferrite grain size after cold rolling and heat treatment is 3.5, 3 and 3 at the 1/16 depth, 1/4 depth and the center of the plate thickness from the surface, respectively. When the soaking temperature was 800 ° C., it was 4.2, 4.6, and 5.0 μm. This grain size is 50 to 60% of the crystal grain size of a commercially available cold-rolled steel sheet having the same composition, about 7.5 μm. An example of the structure of a material having a soaking temperature of 750 ° C. is shown in FIG. The product D · X of the grain growth rate and the particle diameter at 700 ° C. was 0.01 μm ² / min or less, and the particle diameter hardly changed within the measurement time (30 minutes). Further, at a depth of 1/4 of the plate thickness from the surface, 90% or more of the ferrite had a particle size between 1/3 and 3 times the average particle size of the depth. The volume fraction of ferrite was 93% or more for all the steel plates, and the second phase was pearlite. Table 20 shows the mechanical properties of these steel sheets.
It can be seen that the yield strength of the steel of the present invention is increased by 60 to 80 MPa and the tensile strength is increased by 30 to 50 MPa as compared with the commercial steel having the same composition. Uniform elongation (UEL) is about the same as that of commercial steels despite the increase in strength. Although the total elongation EL has decreased, this is due to the fact that the thickness of the commercial steel is as thick as 1.2 mm. Considering the difference in thickness, the strength and elongation balance of the steel of the present invention is the same as or higher than that of the commercial steel. That's it.

  It can be seen that the yield strength of the steel of the present invention is increased by 60 to 80 MPa and the tensile strength is increased by 30 to 50 MPa as compared with the commercial steel having the same composition. Uniform elongation (UEL) is about the same as that of commercial steel despite the increase in strength. Although the total elongation (EL) has decreased, this is because the thickness of the commercial steel is as thick as 1.2 mm, and considering the difference in thickness, the strength and elongation balance of the steel of the present invention is the same as that of the commercial steel. About or above.

  The steel sheet of the present invention has ultrafine crystal grains and is excellent in thermal stability and mechanical properties that can withstand the heat of welding and hot dipping processes. Moreover, such a steel plate excellent in thermal stability and mechanical properties can be easily produced by the method of the present invention.

It is the shape of a ball head projecting specimen. (a) is when the principal strain direction is parallel to the weld line direction, and (b) is when the principal strain direction is perpendicular to the weld line direction. The time change of the ferrite particle diameter in the depth of 1/4 of plate | board thickness from the surface is shown. The change by the annealing time of the ferrite grain size after cold rolling and annealing is shown. The structure after annealing at 800 ° C. for 5 minutes after cold rolling is shown. An example of the structure | tissue after annealing at 750 degreeC after cold rolling is shown.

Explanation of symbols

1 Sphere head projecting specimen 2 Welding line

Claims (14)

In mass%, C: 0.01 to 0.25%, Si: 3% or less, Mn: 3% or less, Al: 3% or less, P: 0.5% or less, Ti: 0 to 0.3%, Nb: 0 to 0.1%, V: 0 to 1%, Cr: 0 to 1%, Cu: 0 to 3 %, Ni: 0 to 1%, Mo: 0 to 1% and Ca + REM + B: 0 to 0. It has a chemical composition consisting of 005% and the balance Fe and impurities, a steel sheet composed of ferrite carbon steel or low alloy steel containing not less than 50 vol%, at 1/4 depth position of the sheet thickness from the steel sheet surface The average crystal grain size D (μm) of ferrite satisfies the following formulas (1) and (2), and is 700 ° C. of the average crystal grain size of ferrite at a depth position ¼ of the plate thickness from the steel sheet surface. The increase rate X (μm / min) and the average crystal grain size D (μm) satisfy the following formula (3): Hot-rolled steel sheet and butterflies.
1.2 ≦ D ≦ 7 (1) Formula D ≦ 2.7 + 5000 / (5 + 350 · C + 40 · Mn) ² (2) Formula D · X ≦ 0.1 (3) Formula where C and Mn are the elements of steel The content (unit: mass%) is shown. At a depth position of ¼ of the plate thickness from the steel sheet surface, the area ratio of the ferrite crystal grains whose crystal grain diameter d (μm) satisfies the following formula (4) is 80% or more. The hot-rolled steel sheet according to claim 1.
D / 3 ≦ d ≦ 3D (4) where D is the average ferrite crystal at a depth of 1/4 of the plate thickness from the steel plate surface. The particle size (μm) is shown. In mass%, C: 0.01 to 0.25%, Si: 3% or less, Mn: 3% or less, Al: 3% or less, P: 0.5% or less, Ti: 0 to 0.3%, Nb: 0 to 0.1%, V: 0 to 1%, Cr: 0 to 1%, Cu: 0 to 3 %, Ni: 0 to 1%, Mo: 0 to 1% and Ca + REM + B: 0 to 0. 005% , a steel plate made of carbon steel or low alloy steel having a chemical composition consisting of Fe and impurities, and containing 50% by volume or more of ferrite, and ferrite at a depth of 1/4 of the plate thickness from the steel plate surface The average crystal grain size D (μm) satisfies the following formulas (5) and (6), and the average crystal grain size of ferrite at a depth position of ¼ of the plate thickness from the steel plate surface at 700 ° C. The increase rate X (μm / min) and the average crystal grain size D (μm) satisfy the following formula (3):
1.2 ≦ D ≦ 9.3 (5) Formula D ≦ 5.0−2.0 · Cr + 5000 / (5 + 350 · C + 40 · Mn) ²・ ・ (6) Formula D ・ X ≦ 0.1 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (3) The ratio of the area occupied by ferrite at the position of the ferrite crystal grains having a crystal grain size d (μm) satisfying the following formula (4) at a depth of 1/4 of the thickness is 80% or more. Cold rolled steel sheet.
D / 3 ≦ d ≦ 3D (4) where C, Cr and Mn are the contents of each element in the steel (unit: mass%) Indicates.   In the steel plate according to any one of claims 1 to 3, as a second phase other than ferrite, the volume ratio is less than 50% bainite, less than 30% pearlite, less than 5% granular cementite, less than 5%. A hot-rolled steel sheet characterized by containing one or two or more selected from the group consisting of martensite and less than 3% retained austenite in a total of less than 50% and having a yield ratio of 0.75 or more, or Cold rolled steel sheet.   The steel sheet according to any one of claims 1 to 3, wherein the second phase other than ferrite contains 5 to 40% martensite by volume and has a yield ratio of less than 0.75. A hot-rolled steel sheet or a cold-rolled steel sheet.   The steel sheet according to any one of claims 1 to 3, wherein the second phase other than ferrite contains 3 to 30% of retained austenite by volume ratio, and has a tensile strength TS (MPa) and total elongation El ( %) And a product TS × El of 18000 (MPa ·%) or more.   Average crystal grain size Ds (μm) at a depth position of 1/16 of the plate thickness from the steel sheet surface, average crystal grain size D (μm) at a depth position of 1/4 of the plate thickness from the steel sheet surface, center of the plate thickness The average crystal grain size Dc (μm) in the portion satisfies a relationship of Ds ≦ 0.75Dc and D ≦ 0.9Dc, according to any one of claims 1, 2, 4, 5 and 6. The hot-rolled steel sheet according to 1.   Satisfying the relationship of Ds ≦ 0.9Dc between the average crystal grain size Ds (μm) at the depth position of 1/16 of the plate thickness from the steel sheet surface and the average crystal grain size Dc (μm) at the center of the plate thickness The cold-rolled steel sheet according to any one of claims 3 to 6, wherein The hot-rolled steel sheet or the cold-rolled steel according to any one of claims 1 to 8, wherein a carbon equivalent Ceq (I) defined by the following formula (7) is 0.06 to 0.6%. Rolled steel sheet.
Ceq (I) = C + Mn / 6 + Si / 24 + Cr / 5
+ Mo / 4 + Ni / 40 + V / 14 (7) Formula Here, the element symbol in the formula indicates the content (unit: mass%) of each element in the steel. The C content is 0.17% by mass or less, the carbon equivalent Ceq (II) defined by the following formula (8) is 0.03 to 0.20%, and the following formula (9) The hot-rolled steel sheet or cold-rolled steel sheet according to any one of claims 1 to 8, wherein the index Rsp of the base material resistance defined by the formula (1) is 45 or less.
Ceq (II) = C + Mn / 100 + Si / 90 + Cr / 100 (8) Formula Rsp = 13.5 × (Si + Al + 0.4Mn + 0.4Cr) +12.2 (9) where the element symbol in the formula Indicates the content of each element in steel (unit: mass%).   A hot-rolled hot-rolled steel sheet or cold-coated steel sheet, comprising a coating layer of Zn, Al, Zn-Al alloy or Fe-Zn alloy on the surface of the hot-rolled steel sheet according to any one of claims 1 to 10. Rolled steel sheet. A method of producing a hot-rolled steel sheet by multi-pass hot rolling a slab made of carbon steel or low alloy steel, and ending the final rolling pass at a temperature of Ar ₃ points or higher and 780 ° C. or higher, and then 400 ° C. After cooling to 720 ° C. or less within 0.4 seconds at a cooling rate of at least / sec, the temperature is maintained at 600 to 720 ° C. for 2 seconds or more, A method for producing a hot-rolled steel sheet according to any one of 6, 7, 9, 10 and 11.   The hot-rolled steel sheet obtained by the method according to claim 12 is pickled, cold-rolled at a rolling rate of 40 to 90%, and then heat-treated at a temperature of 900 ° C or lower. The manufacturing method of the cold-rolled steel plate in any one of 4, 5, 6, 8, 9, and 10.   The hot-rolled steel sheet obtained by the method according to claim 12 is pickled or cold-rolled at a rolling rate of 40 to 90% after pickling and then hot-plated in a continuous hot dipping line. The method for producing a hot-dip hot-rolled steel sheet or hot-rolled cold-rolled steel sheet according to claim 11.

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